The U.S. Pat. No. 4,311,980 discloses a pressure transducer and a method for making it. A plurality of thickfilm resistors are screen-printed onto a deformable substrate specifically a circular diaphragm formed of ceramic material and clamped along its periphery. The resistors are circuit-connected to a Wheatstone bridge for measuring pressure or forces applied to the diaphragm. One problem of this known transducer is the assembly and safe securement of the diaphragm in mass production. Furthermore, the thickfilm resistors are subject to exterior influences leading to considerable deterioration. In many cases specific measures are required for securing the known transducer in a fixed position. Even then it is almost impossible to avoid a distortion of the diaphragm resulting in considerable non-linearities of the measurement.
The journal "messen prufen automatisieren", June 1986, pages 340, 341 and July/August 1987 pages 435 to 437 discloses a pressure transducer using a diaphragm as explained above. Instead of clamping the diaphragm peripherally the diaphragm of this known transducer is secured to a rigid substrate by a circular glass layer leaving a cavity in the central part. This means that after heat treatment of the diaphragm another treatment of the whole unit is necessary in order to melt the glass frit. Depending on the quantity of glass frit used for forming the circular ring quite different cavities will result. For evacuation or connection to the atmosphere it is necessary to provide the substrate with a bore. This publication shows the application of a hybrid circuitry on the rear side of the substrate and the incorporation of such a transducer in a transducer housing.
The German Patent Publication No. 12 99 441 shows a flat pressure transducer including two opposing plates maintained in spaced relationship by a rectangular frame. At least one of the opposing inner surfaces of the two plates is provided with a strain gauge.
The German Patent No. 11 25 205 discloses a pressure transducer using a measuring member projecting with air-tight sealing from a housing. For compensating the influence of the atmospheric pressure the measuring member is set under atmospheric pressure from the opposite side of the projection.
The German Laid Open Publication No. 36 31 651 discloses an acceleration or vibration transducer using a circular diaphragm peripherally clamped between two rigid parts forming an air-tight cavity. At a central position the diaphragm is provided with a mass element such that upon application of an acceleration or vibration onto the unit the diaphragm is bent in the one or other direction resulting in a change of resistance of thickfilm resistors applied to one surface of the diaphragm by screen printing. With a principal illustration a canty-lever type of acceleration or vibration transducer is disclosed having the free end thereof provided with a mass element and one surface provided with a thickfilm resistor circuitry. Again, the practical embodiment with the clamped diaphragm has the disadvantages as outlined in connection with the pressure transducer according to U.S. Pat. No. 4,311,980. No practical embodiment is shown for the canty-lever type acceleration and vibration transducer.
The German Laid Open Publication No. 33 10 538 discloses a pressure transducer similar to that of the U.S. Pat. No. 4,311,980, however, with a rectangular shape of the diaphragm and the clamping elements.